Setting too big icon for custom type can break the editor

[KoBeWi]

Godot version

3.5 beta1 and probably master

System information

W10

Issue description

None of the icons adjust texture's size, so using too big texture results in this abomination.

"Fixing" this is probably unfeasible, so at least EditorPlugin.add_custom_type() description should mention a standard size for an icon.

Steps to reproduce

1. Have EditorPlugin with add_custom_type()
2. Use some very big texture for the icon

Minimal reproduction project

No response

[Calinou]

Related to #20197.

[fire]

Hope that this interacts with hidpi where the size might be 3x the normalized pixel size.

[KoBeWi]

Ok I just discovered that, just like #20197, it doesn't happen with class_name.

[akien-mga]

Another example :)

We "just" need to enforce a max size I believe. I.e. resizing the user provided icon as needed.